Bit stream conversion system

ABSTRACT

A bit-stream converter capable of converting a first synchronous compressed bit-stream of data at a first sampling rate to second synchronous compressed bit-stream frame of data at a second sampling rate is disclosed. The bit-stream converter architecture may include a payload length detector and a zero stuffing unit in signal communication with the payload length detector. The zero stuffing unit is capable of zero stuffing section responsive to the payload length detector detecting the payload length.

FIELD OF THE INVENTION

[0001] This invention relates generally to the field of datacommunications. In particular, the invention relates to datacommunication systems that utilize compressed bit-streams.

RELATED ART

[0002] The complexity of consumer video and audio electronics componentsystems is increasing at rapid pace. Systems such as compact disk(“CD”), laserdisc, digital video disc or digital versatile disc (“DVD”),mini-disk, and others are now common. As a result, a modern trend is tointegrate all these systems into home theater and automotiveentertainment systems.

[0003] Generally, current CD and DVD chipsets provide a Sony/PhilipsDigital Interface (“S/PDIF”) that outputs audio in bit-streams of dataaccording to the ISO/IEC 60958 (i.e., linear pulse code modulation(“PCM”)) and ISO/IEC 61937 (i.e., non-linear PCM) standards. Typically,compressed multi-channel audio bit-streams, such as Dolby Digital®(AC-3), DTS®, MLP®, MP3®, MPEG II®, MPEG II-AAC® etc. are formattedaccording to ISO/IEC 61937 and are conveyed over S/PDIF to an externalaudio decoder. The bit-streams of data are bi-phased coded with a symbolfrequency of 64 times (for very low bit-rates 128 times) the originalsampling-rate (“f_(sample)”). A S/PDIF receiver typically locks on thebit-stream and synchronously generates the f_(sample) for decoding thebit-stream of data. The sampling frequency of the original bit-streammay cover the range of 8-192 kHz (e.g., CD are typically 44.1 kHz, DVD-Vtypically 48 kHz, DVD-A typically 96 kHz, etc.).

[0004] Multimedia networks may be utilized to integrate CD and DVD typecomponents into modern home theater and automotive entertainmentsystems. Unfortunately, many multimedia networks operate in asynchronous manner at a constant rate of e.g. 44.1kHz that is differentthan the encoded audio source. In order to transport digital audio froma digital source (such as a CD or DVD) to a destination (such as adecoding amplifier) over the synchronous channels, the audio samplingrate (e.g. 48 kHz for a DVD) needs to be adapted to the multimedianetwork sampling rate (44.1 kHz). A previous approach to adapt the twosampling rates includes sample rate converting the audio. However, sincecompressed multi-channel audio is a bit-stream rather than pulse codemodulation (“PCM”) samples, this approach cannot be applied immediately.The audio needs to be decoded first into typically 5.1 PCM channels andthen sample rate conversion may be applied prior to sending it overmultimedia network. Decoded audio, however, occupies much more bandwidththan the compressed bit-stream. Therefore, there is a need for a systemthat is capable of adapting the two sampling rates.

SUMMARY

[0005] This invention provides a bit-stream converter capable ofconverting a first synchronous compressed bit-stream frame of data at afirst sampling rate to a second synchronous compressed bit-stream frameof data at a second sampling rate. Such a bit-stream converter mayutilize a system architecture that performs a process for converting afirst synchronous compressed bit-stream frame of data at a firstsampling rate to a second synchronous compressed bit-stream frame ofdata at a second sampling rate. The process may include determining aformat for the first compressed bit-stream frame. The first compressedbit-stream frame may have a frame length and may include a data-burstsection and a stuffing section. The data-burst section may have apayload section including a preamble section and a payload length, whilethe stuffing section may have a stuffing length. The process may alsoinclude zero stuffing the stuffing section in response to a particularformat.

[0006] The bit-stream converter architecture may include a payloadlength detector and a zero stuffing unit in signal communication withthe payload length detector. The zero stuffing unit is capable of zerostuffing the stuffing section responsive to the payload length detectordetecting the payload length.

[0007] This invention also provides an inverse bit-stream converter forconverting a first synchronous compressed bit-stream frame of data at afirst sampling rate having zero stuffing to a second synchronouscompressed bit-stream frame of data at a second sampling rate. Thebit-stream converter may include a synchronization unit, a formatdetector in signal communication with the synchronization unit and azero stuffing removal unit in signal communication with format detector.The format detector may be capable of determining a format for the firstsynchronous compressed bit-stream frame of data.

[0008] Other systems, methods, features and advantages of the inventionwill be or will become apparent to one with skill in the art uponexamination of the following figures and detailed description. It isintended that all such additional systems, methods, features andadvantages be included within this description, be within the scope ofthe invention, and be protected by the accompanying claims.

BRIEF DESCRIPTION OF THE FIGURES

[0009] The components in the figures are not necessarily to scale,emphasis instead being placed upon illustrating the principles of theinvention. In the figures, like reference numerals designatecorresponding parts throughout the different views.

[0010]FIG. 1 is a block diagram illustrating an example implementationof bit-stream conversion system.

[0011]FIG. 2 is a block diagram illustrating an example format of abit-stream.

[0012]FIG. 3 is a block diagram illustrating an example implementationof the bit-stream converter element of FIG. 1.

[0013]FIG. 4 is a block diagram illustrating an example implementationof the inverse bit-stream converter element of FIG. 1.

[0014]FIG. 5 is a flowchart illustrating an example process preformed bythe bit-stream converter of FIG. 3.

[0015]FIG. 6 is a flowchart illustrating an example process preformed bythe inverse bit-stream converter of FIG. 4.

DETAILED DESCRIPTION

[0016]FIG. 1 shows a multimedia data communication system 100 includingthe bit-stream conversion system 102. The bit-stream conversion system102 may include a bit-stream converter 104 in signal communication witha source 106 and a network 108. The inverse bit-stream converter 110 maybe in signal communication with the network 108 (such as a multimedianetwork) and a decoder 112.

[0017] The source 106 may be a compact disk (“CD”) or derivativeproduct, a mini-disc or derivative product, a digital video disc ordigital versatile disc (“DVD”) or derivative product, or otherequivalent type sources. The network 108 may be any link or network(wireless or physical link) that provides a clock (i.e. being clockmaster) that is different from the S/PDIF source.

[0018] The bit-stream conversion system 102 converts compressedbit-streams of data from the source 106 to match the network 108sampling rate (also known as the transport rate) without altering theaudio information in the bit-stream converter 104. The inversebit-stream converter 110 then receives converted compressed bit-streamsof data from the network 108 and determines the original samplingfrequency f_(sample) 114 and outputs new bit-stream of data 116 that isa reproduction of the bit-stream of data produced by the source 106. Thenew bit-stream of data 116 is input into the decoder 112 and the decoder112 decodes the new bit-stream of data 116 producing separate pulsecoded modulation (“PCM”) channels that may be transmitted to a receivervia signal path 120.

[0019]FIG. 2 is a block diagram illustrating an example format of abit-stream 200. The bit-stream 200 may include numerous frames 202. Eachframe 202 may include sub-frames such as a data-burst section 204 andstuffing section 206. The data-burst section may include a preamble andpayload section 208. The preamble may include header information such asPa 210, Pb 212, Pc 214 and Pd 216. Pa may equal 0×F872 and Pb may equal0x4E1F. Both Pa and Pb represent a synchronization word that indicatesthe start of the data burst and may be utilized to obtain the samplingrate f_(sample). Pc represents the burst information and indicates thetype of data in the bit-stream and some information and/or control forthe receiver (not shown). Pd represents the length of the burst-payloadin bits. The frame 202 has a period T_(period) 218.

[0020] As an example, if the network 108 is designed to transmit CDaudio signals, the network 108 may operate with a sampling frequencyf_(sample) of approximately 44.1 kHz and may be designed to transmit twochannel linear PCM signals at 44.1 kHz. If the source 106 is a DVD,instead of a CD, the source 106 may transmit bit-streams of data thatinclude multi-channel audio signals. These multi-channel audio signalsmay be compressed such that their transmission rate is lower than theirequivalent 2-channel PCM version. In this methodology, multi-channelaudio can be transmitted utilizing less than or equal to the samechannel bandwidth of linear stereo PCM. As a result, the data length ofthe payload section of the DVD signal will be shorter than theequivalent data length of the payload section of a CD signal. Therefore,in order to maintain the same transmission signal period T_(period) 218between the DVD and CD signals, zero stuffing may be utilized to expandthe length of the stuffing section 206 in order to compensate for theshorter payload section 208.

[0021] For example, IEC 61937 specifies how non-linear PCM (compressedaudio) is transferred over S/PDIF. S/PDIF is a unidirectional bi-phasedcoded link and there is no handshake between source 106 and thedestination. The compressed audio frame always represents a constantnumber of samples, (1536 for Dolby Digital® AC-3). According to thecompression rate, the actual data burst may be shorter (i.e., a highcompression rate) or longer (i.e., a low compression rate). However,since there is no handshaking in S/PDIF, the process clocks out the dataframe, which in this example is 1536×(64× sampling frequency) clockperiods, before the next data burst is sent. Since the payload is lowerthan 1536×(64×f_(sample)), the rest is filled with zero bits (“zerostuffing”).

[0022] By reducing and/or stretching the zero-stuffing, theburst-payloads may be transported at a different data rate withoutaffecting the payload. In a typical Dolby Digital® bit-stream thesampling frequency is 48 kHz and the compression rate is 448 kbps. Onecompressed Dolby Digital® frame always represents 1536 samples. Theoriginal repetition rate between 2 data bursts is, therefore, 1536/48kHz=32 ms. If the network is operating at 44.1 kHz, the repetition rateequivalently needs to be reduced to 1411.2 in order not to loose anyinformation (1411.2/44.1 kHz=32 ms). Consequently, the amount ofzero-stuffing should be reduced by 124.8 IEC 60958/61937 frames. Because1411.2 is a rational number, the goal is to reduce the stuffing of 4consecutive burst-payloads by 125 frames (1411) and the 5^(th)burst-payload by 124 IEC 60958/61937 frames (1412), such that theaverage data rate of 1411.2 is respected.

[0023] In this example, the original frame repetition rate is 32 ms(burst-payload and stuffing). However, for a network clock (e.g. 44.1kHz) that is lower than the source clock (e.g. 48 kHz), less bits needto be transported before starting the next frame. Therefore, the amountof zeros should be reduced because it does not affect the payload. Theamount of reduction is represented by the relation of 48/44.1. Becausethis relation is not an integer, an approach is applied that is similarto a leap-year correction. Here, every fifth frames is slightly longerso that the average frame rate remains 32 ms. If the source is at alower f_(sample) (say 32 kHz) than the network, then the amount of zeroshas to be increased (stretched) correspondingly.

[0024] For other formats, the compression may be relatively low. Forexample, the DTS format has 6 IEC 60958/61937 zero-frames availablebetween 2 burst-payloads. This is less than required for bit-streamconversion from 48 kHz to 44.1 kHz. Therefore, in this example, a 2^(nd)stereo transport channel may be utilized to transport all information at44.1 kHz (assume DTS 48 kHz, bit-rate=1509.75 kbps). The following tablesummarizes some typical examples: IEC 61937 Repetition Network PreamblePeriod Bit-rate (kbps)/ Repetiton Period Format Pc Frames (bytes)Payload (bytes) for 44.1 kHz AC-3 (48 kHz) 1 1536 (6144) (32-640)/1411.2 (4x1411 + 1x1412) (128-2560) MP3 (32 kHz) 5 1152 (4608) 1587.6(4x1588 + 1x1586) MP3 (44.1 kHz) 5 1152 (4608) 1152 MP3 (48 kHz) 5 1152(4608) 1058.4 (4x1058 + 1x1060) AAC (48 kHz) 7 1024 (4096) 940.8(4x941 + 1x940 DTS I (48 kHz) 11  512 (2048)  754.50/1006 470.4 (4x470 +1x472) DTS II (44.1 kHz) 12 1024 (4096) 1234.00/4096 1024 DTS I (48 kHz)11  512 (2048) 1509.75/2013 470.4 (4x470 + 1x472) DTS III (24/96) 132048 (8192) 940.8 (4x941 + 1x940)

[0025]FIG. 3 is a block diagram illustrating an example implementationof the bit-stream converter 104. The bit-stream converter 104 mayinclude a synchronization unit 300, a frequency detector 302, a payloadlength detector 304, a zero stuffing unit 306 and a counter 308. Thesynchronization unit 300 is in signal communication with the source 106via a signal path 310. The synchronization unit 300 is also in signalcommunication with frequency detector 302 and counter 308. Frequencydetector 302 is in signal communication with both the synchronizationunit 300 and the payload length detector 304. The zero stuffing unit 306is in signal communication with the payload length detector 304, thecounter 308 and the network 108 via signal path 312. The counter 308 maybe a modulo-N counter (in this case N=5).

[0026] In operation, the synchronization unit 300 (also known as “SYNC”)identifies the preamble Pa, Pb, Pc and Pd of a new burst-payload. TheSYNC compares the bit-stream to the preamble Pa and Pb, and if a matchis found, the SYNC triggers the modulo-N (here N=5) counter for correctzero stuffing modification at the zero stuffing unit 306. Pc may act toidentify the type of encoding. The SYNC reads the Pc, the frequencydetector 302 detects the sampling frequency and the length of thepayload is determined by the payload length detector (from reading Pd)in order to modify the zero stuffing by the zero stuffing unit 306. Thismethodology also may determine how many network channels need to beallocated in parallel.

[0027]FIG. 4 is a block diagram illustrating an example implementationof the inverse bit-stream converter 110. The inverse bit-streamconverter 110 may include a synchronization unit 400, a format detector402, a frequency detector 404 and a phase lock loop (“PLL”) 406. Thesynchronization unit 400 is in signal communication with the network108, the format detector 402 and the frequency detector 404. Thefrequency detector 404 is in signal communication with the PLL 406 andthe decoder 112.

[0028] In operation, the inverse bit-stream converter 110 extracts theoriginal bit-stream information and triggers the PLL 406 to generatesynchronously the original sampling frequency 114. For example, if thenetwork clock 420 is operating at 44.1 kHz but the frequency detector404 detects that the original bit-stream is at 48 kHz, the PLL 406 isdriven by the network clock 420 and the frequency detector 404 torecover the 48 kHz required by the decoder 112. The decoder 112 uses theparameters from 402 and 404 to properly decode the audio in order toproduce and output a signal via signal path 120.

[0029] A controller (not shown) may be utilized to control the operationof the bit-stream converter 104 and inverse bit-stream converter 110.The controller may be any type of control device that may be selectivelyimplemented in software, hardware (such as a computer, processor, microcontroller or the equivalent), or a combination of hardware andsoftware. The controller may utilize optional software (not shown).

[0030] The software, includes an ordered listing of executableinstructions for implementing logical functions, may selectively beembodied in any computer-readable (or signal-bearing) medium for use byor in connection with an instruction execution system, apparatus, ordevice, such as a computer-based system, processor-containing system, orother system that may selectively fetch the instructions from theinstruction execution system, apparatus, or device and execute theinstructions. In the context of this document, a “computer-readablemedium” and/or “signal-bearing medium” is any means that may contain,store, communicate, propagate, or transport the program for use by or inconnection with the instruction execution system, apparatus, or device.The computer readable medium may selectively be, for example but notlimited to, an electronic, magnetic, optical, electromagnetic, infrared,or semiconductor system, apparatus, device, or propagation medium. Morespecific examples “a non-exhaustive list” of the computer-readablemedium would include the following: an electrical connection“electronic” having one or more wires, a portable computer diskette(magnetic), a RAM (electronic), a read-only memory “ROM” (electronic),an erasable programmable read-only memory (EPROM or Flash memory)(electronic), a Magnetic Random Access Memory (“RAM”), a Ferro RandomAccess Memory (“FRAM”), a chalcogenide memory or Ovonic Universal Memory(“OUM”), a polymer memory, a MicroElectroMechanical ( MEMS”) memory anda write once 3D memory, an optical fiber (optical), and a portablecompact disc read-only memory “CDROM” (optical). Note that thecomputer-readable medium may even be paper or another suitable mediumupon which the program is printed, as the program can be electronicallycaptured, via for instance optical scanning of the paper or othermedium, then compiled, interpreted or otherwise processed in a suitablemanner if necessary, and then stored in a computer memory.

[0031]FIG. 5 is a flowchart 500 illustrating an example processperformed by the bit-stream converter 104. This process may be performedby hardware, software or combination of both. The process starts 502with the input reception 504 of information such as a bit-stream of databy bit-stream converter 104. The synchronization unit 300 determines thepreamble values Pa, Pb, Pc and Pd 506. In decision 508, a comparatorunit (not shown) within the synchronization unit 300 compares thebit-stream to the preamble parameters Pa and Pb 508. If the result inthe decision 508 is not an approximate match between the bit-stream andpreamble values Pa and Pb, the process returns 509 to step 504 andrepeats.

[0032] If instead the result of decision 508 is an approximate matchbetween the bit-stream and preamble values Pa and Pb, the counter isstarted 510 and the sampling frequency of the bit-stream is determined512. Next, the payload length detector 304 determines the payload length514. Next, the zero stuffing unit stuffs the stuffing section with theappropriate number of zeros 516 and the process ends at step 518.

[0033]FIG. 6 is a flowchart 600 illustrating an example processperformed by the inverse bit-stream converter 110. The example processmay be performed by hardware, software or combination of both. Theprocess starts at step 602 with the input and reception of thebit-stream data 604 by the inverse bit-stream converter 110. Thesynchronization unit 400 determines the preamble values Pa, Pb, Pc andPd 606. In decision 608, a comparator unit (not shown) within thesynchronization unit 400 compares the bit-stream to the preambleparameters Pa and Pb. If the result in decision step 608 is not anapproximate match between the bit-stream and preamble values Pa and Pb,the process returns 610 to step 604 and repeats.

[0034] If instead the result of decision 608 is an approximate matchbetween the bit-stream and preamble values Pa and Pb, the formatdetector determines the format type Pc of the bit-stream 612. Thefrequency detector then determines the original sampling frequency ofthe compressed audio 614. Next, the decoder 112 decodes the bit-stream616 and produces an output signal that is transmitted to a receiver,e.g. a digital to analog converter (not shown). The PLL locks on to thesampling frequency of the bit-stream 618 and produces the originalfrequency rate 114 of the original bit-stream. The process then ends instep 620.

[0035] While various embodiments of the invention have been described,it will be apparent to those of ordinary skill in the art that many moreembodiments and implementations are possible that are within the scopeof this invention.

What is claimed is:
 1. A method for converting a first synchronouscompressed bit-stream frame of data at a first sampling rate to secondsynchronous compressed bit-stream frame of data at a second samplingrate, the method comprising: determining a format for the firstcompressed bit-stream frame where the first compressed bit-stream framehas a frame length and the first compressed bit-stream frame includes adata-burst section having preamble section and a payload section havinga payload length and a stuffing section having a stuffing length; andzero stuffing the stuffing section responsive to determining the format.2. The method of claim 1, wherein determining a format includesdetermining the payload length.
 3. The method of claim 2, wherein zerostuffing includes zero stuffing the stuffing section responsive todetermining the payload length.
 4. The method of claim 3, wherein thestuffing length is equal to the frame length minus the payload length.5. The method of claim 1, wherein determining a format includesdetermining the format from the preamble section.
 6. The method of claim5, further including receiving the first synchronous compressedbit-stream frame of data, determining a synchronization word and payloadlength from the preamble section, comparing the first synchronouscompressed bit-stream frame of data to the synchronization word,starting a counter in response to the comparison, and determining firstsampling rate.
 7. The method of claim 6, wherein zero stuffing includeszero stuffing the stuffing section responsive to determining the payloadlength.
 8. The method of claim 7, wherein the stuffing length is equalto the frame length minus the payload length.
 9. A method for convertinga first synchronous compressed bit-stream frame of data at a firstsampling rate having zero stuffing to second synchronous compressedbit-stream frame of data at a second sampling rate, the methodcomprising: determining a format for the first compressed bit-streamframe where the first compressed bit-stream frame has a frame length andthe first compressed bit-stream frame includes a data-burst sectionhaving preamble section and a payload section having a payload lengthand a stuffing section having a stuffing length; and removing zerostuffing from the stuffing section responsive to determining the format.10. The method of claim 9, wherein determining a format includesdetermining the payload length.
 11. The method of claim 10, whereinremoving includes removing the zero stuffing from the stuffing sectionresponsive to determining the payload length.
 12. The method of claim11, wherein the stuffing length is equal to the frame length minus thepayload length.
 13. A bit-stream converter for converting a firstsynchronous compressed bit-stream frame of data at a first sampling rateto second synchronous compressed bit-stream frame of data at a secondsampling rate, the bit-stream converter comprising: a payload lengthdetector; and a zero stuffing unit in signal communication with thepayload length detector, the zero stuffing unit capable of zero stuffingthe stuffing section responsive to the payload length detector detectingthe payload length.
 14. The bit-stream converter of claim 13 furtherincluding a synchronization unit; a frequency detector in signalcommunication with the synchronization unit and payload length detector;and a counter in signal communication with the synchronization unit andzero stuffing unit.
 15. An inverse bit-stream converter for converting afirst synchronous compressed bit-stream frame of data at a firstsampling rate having zero stuffing to second synchronous compressedbit-stream frame of data at a second sampling rate, the bit-streamconverter comprising: a synchronization unit; a format detector insignal communication with the synchronization unit, the format detectorcapable of determining a format for the first synchronous compressedbit-stream frame of data; and a zero stuffing removal unit in signalcommunication with format detector.
 16. A bit-stream converter forconverting a first synchronous compressed bit-stream frame of data at afirst sampling rate to second synchronous compressed bit-stream frame ofdata at a second sampling rate, the bit-stream converter comprising:means for determining a payload length; and means for zero stuffing astuffing section responsive to determining the payload length.
 17. Thebit-stream converter of claim 16 further including means forsynchronization; a frequency detector in signal communication with thesynchronization means and the determining means; and a counter in signalcommunication with the synchronization means and zero stuffing means.18. An inverse bit-stream converter for converting a first synchronouscompressed bit-stream frame of data at a first sampling rate having zerostuffing to second synchronous compressed bit-stream frame of data at asecond sampling rate, the bit-stream converter comprising: means forsynchronization; means for determining a format for the firstsynchronous compressed bit-stream frame of data; and means for removingthe zero stuffing.
 19. A signal-bearing medium having software forconverting a first synchronous compressed bit-stream frame of data at afirst sampling rate to second synchronous compressed bit-stream frame ofdata at a second sampling rate, the signal-bearing medium comprising:logic configured for determining a format for the first compressedbit-stream frame where the first compressed bit-stream frame has a framelength and the first compressed bit-stream frame includes a data-burstsection having preamble section and a payload section having a payloadlength and a stuffing section having a stuffing length; and logicconfigured for zero stuffing the stuffing section responsive todetermining the format.
 20. The signal-bearing medium of claim 19,wherein determining logic includes logic configured for determining thepayload length.
 21. The signal-bearing medium of claim 20, wherein zerostuffing logic includes logic configured for zero stuffing the stuffingsection responsive to the logic configured for determining the payloadlength.
 22. The signal-bearing medium of claim 21, wherein the stuffinglength is equal to the frame length minus the payload length.
 23. Thesignal-bearing medium of claim 19, wherein determining logic includeslogic configured for determining the format from the preamble section.24. The signal-bearing medium of claim 23, further including logicconfigured for receiving the first synchronous compressed bit-streamframe of data, logic configured for determining a synchronization wordand payload length from the preamble section, logic configured forcomparing the first synchronous compressed bit-stream frame of data tothe synchronization word, logic configured for starting a counter inresponse to the comparison, and logic configured for determining firstsampling rate.
 25. The signal-bearing medium of claim 24, wherein zerostuffing logic includes logic configured for zero stuffing the stuffingsection responsive to determining the payload length.
 26. Thesignal-bearing medium of claim 25, wherein the stuffing length is equalto the frame length minus the payload length.
 27. A signal-bearingmedium having software for for converting a first synchronous compressedbit-stream frame of data at a first sampling rate having zero stuffingto second synchronous compressed bit-stream frame of data at a secondsampling rate, the signal-bearing medium comprising: logic configuredfor determining a format for the first compressed bit-stream frame wherethe first compressed bit-stream frame has a frame length and the firstcompressed bit-stream frame includes a data-burst section havingpreamble section and a payload section having a payload length and astuffing section having a stuffing length; and logic configured forremoving zero stuffing from the stuffing section responsive todetermining the format.
 28. The signal-bearing medium of claim 27,wherein the determining logic includes logic configured for determiningthe payload length.
 29. The signal-bearing medium of claim 28, whereinremoving logic includes logic configured for removing the zero stuffingfrom the stuffing section responsive to determining the payload length.30. The signal-bearing medium of claim 29, wherein the stuffing lengthis equal to the frame length minus the payload length.